The ability to sense environmental temperature is essential for human life. Mammals detect temperature cues via thermosensitive receptors expressed in sensory nerve endings and keratinocytes in the skin. Work in the past fifteen years has identified five TRP channels as the primary heat receptors that sense a full range of warm and hot temperatures, spanning from 33C to over 53C. This has led to a fairly clear understanding of how animals sense heat. By contrast, our understanding of how animals sense cold temperatures is far from complete. Two TRP channels (TRPM8 and TRPA1) are found to respond to cold; however, they only modestly contribute to cold sensation. Apparently, cold-sensitive channels other than TRPs, particularly those sensing noxious cold temperatures (<15C), must exist but have yet to be identified. The difficulty largely results from the lack of n efficient screening system for identifying temperature-sensitive channels. The identification of those TRP channels as thermo-receptors was all achieved through candidate gene approaches, but none of the remaining TRP channel members has been found to be cold-sensitive. Thus, the unknown cold receptors must be encoded by distinct classes of genes. Similar to mammals, C. elegans also senses a full range of temperature cues through thermosensitive channels. Interestingly, most, if not all, ion channels (e.g. voltage-, mechanically-, temperature-, and ligand-gated channels) are evolutionarily conserved in C. elegans. In particular, those known thermosensitive channels, such as TRP channels, are also found in C. elegans. Thus, thermosensitive channels in C. elegans are likely to be evolutionarily conserved. This, together with its short generation time (~3 days) and facile and rich genetic tools, makes C. elegans an ideal system for identifying novel thermosensitive channels. To circumvent the difficulty surrounding the cloning of the elusive cold- sensitive channels, here we develop C. elegans as a novel high throughput in vivo screening system. Using this system, we have isolated mutants defective in cold sensation. In this proposal, we will first clone mutant genes and then characterize these genes and their mammalian homologs in heterologous systems to test whether they encode cold-sensitive channels. The proposed work may lead to identification of the elusive thermosensitive channels that detect noxious cold and mediate cold-evoked pain and thermoregulation in mammals. Moreover, as thermosensitive channels are also activated by other noxious cues such as noxious chemicals and/or mechanical stimuli, these channels are broadly involved in pain sensation. As such, the proposed work may also facilitate our understanding of pain sensation as a whole and may help identify potential therapeutic targets for pain treatment.